A Comparison of Ultrasound and MRI Measurements of the Cross Sectional Area of the Median Nerve at the WristJennifer D'Auria, MD; Beverly Lida Hersh, BA; John R. Fowler, MDUniversity of Pittsburgh Medical Center, Pittsburgh, PA

Background: Ultrasound (US) measurement of the cross-sectional area (CSA) of the median nerve has emerged as a viable alternative to the use of electromyography/nerve conduction studies (EMG/NCS) for the diagnosis of carpal tunnel syndrome (CTS).1 In a classic study, Buchberger et al (1992).2compared the CSA of the median nerve using magnetic resonance imaging (MRI) and US. The authors found US to be an accurate method to measure the CSA of the median nerve, but did not specifically report the mean difference between US and MRI measurements and did not provide the strength of correlation. The purpose of this study is to compare CSA of the median nerve between US and MRI using current MRI and US technology. The null hypothesis is that there is no difference between US and MRI CSA measurements.

Methods: The study was designed as an observational cohort, enrolling patients who presented to clinic for evaluation with previous MRI of the wrist (for a diagnosis other than carpal tunnel syndrome). Subjects witih clinical signs and symptoms of carpal tunnel syndrome were excluded. MRI measurements of CSA of the median nerve on T1 weighted axial images of the wrist were performed by a hand fellow blinded to the results of US measurements and US measurement of median nerve CSA was performed by a hand fellowship trained surgeon blinded to the results of the MRI measurements. The measurements were analyzed via percent error, Pearson correlation, and t-tests.

Results: Twenty subjects were enrolled, with mean age 29.4. Four left wrists and sixteen right writsts were measured. Diagnoses included, but were not limited to, scaphoid fracture, ganglion cyst, synovitis, and triangular fibrocartilage complex tear. Mean CSA of the median nerve on US was 6.8mm2 (+/- 2.330 mm2), and mean CSA of the median nerve on MRI was 6.8 mm2 (+/- 2.153 mm2), P = 0.442. Pearson correlation coefficient between modalities was 0.93, suggesting near perfect correlation.3 Mean percent error was 8.8%.

Conclusions: Results of this study confirm the work of Buchberger and colleagues and suggest that US is an accurate method to measure the CSA of the median nerve at the carpal tunnel inlet. The mean difference between US and MRI was small and unlikely to be of clinical significance.